NCEIL-4: Meet a new electron-injection material and its capabilities
Article by Alicja Zielińska, Product Manager at Noctiluca
At Noctiluca, we are focused on creating materials for the products of the future. Primarily, we conduct research on 3rd, 4th and 5th generation emitters for use in the optoelectronic industry. One of the areas of focus is enabling the development of printed electronics, with particular emphasis on inkjet printing technology. As a chemical company, we closely follow the needs of our industry and propose new solutions that improve the parameters of currently used 1st and 2nd generation OLED devices.
With the race for blue emitter still on (at length described HERE), our team has asked ourselves, if there is another way to improve the OLED stack and overall lifetime of blue pixel. Once again, we turned our attention to the needs of the market and pain points of our clients. It was within the scope of such targeted research and collaboration with entities involved in OLED manufacturing (under chemical CRO/CDMO model) that a new electron-injection material (EIL), NCEIL-4, was developed.
Studies, tests and patent application
Based on the studies we conducted in comparison to the commonly known and used material Liq (8-Quinolinolato lithium), we produced blue OLEDs characterized by:
- a significant improvement in device lifetimes (almost 5 times higher LT90),
- enhanced EQE and current efficiency, while maintaining high color quality of the emitted blue light,
- reduced driving voltage with the maintenance of low turn-on voltage.
The causes of such significant improvements in certain parameters include:
- better electron injection in the low-field region of the devices,
- lower charge injection barrier between the cathode and electron transport layer compared to devices with Liq as the EIL,
- as well as better charge transport in devices containing NCEIL-4 in their structure.
Based on literature reports, we decided to test the impact of NCEIL-4 as an n-dopant of the electron transport layer. In previous tests, we used PBPPhen as the ETL. The optimal doping level, as well as the thickness of the layer, were determined based on the results for the fabricated diodes.
The most important advantages of introducing the n-dopant, which in this case is NCEIL-4, include:
- a threefold increase in lifetime compared to devices where the ETL was not doped, and nearly 15 times longer for devices where Liq served as the EIL and PBPPhen was not doped with our material,
- reduced dependence of the EQE parameter on the applied luminance (lower roll-off), leading to more stable devices across a wider operating range.
Seeing the potential of NCEIL-4, we decided to create a series of products for OLED applications, including materials for electron injection or transport. A patent application has also been filed based on this development in 2024.
15 times longer lifetime
To better demonstrate the capabilities of NCEIL-4 in OLED devices, we conducted a series of tests. In ref.1, the EIL layer is Liq, while in stacks 1 – 4, our proprietary compound NCEIL-4 was used, with different layer thicknesses ranging from 1.0 nm to 1.5 nm. The difference between stacks 1-2 and stacks 3-4 is that the ETL in 3-4 was additionally doped with NCEIL-4. The results? The lifetime of devices 1 and 2 is 5 times longer when using NCEIL-4 as the EIL layer, and when we doped the ETL layer with NCEIL-4, the lifetime is 15 times longer.
Broad applications
Due to the similarity of NCEIL-4 to compounds like Liq and LiF, there is a possibility of replacing them in OLED stacks with Noctiluca’ materials from the NCEIL series. Below there just a sample list of applications where NCEIL-4 can be utilized:
1. OLEDs prepared using PVD techniques, as well as wet methods like inkjet printing:
- Electron transport layer (ETL), improving electron injection and luminance efficiency.
- Buffer layer (EIL) between the cathode and organic layer, enhancing electron injection efficiency.
2. Organic Photovoltaic Cells (OPV):
- Electron transport layer, reducing energy losses at the cathode-active layer interface.
- Buffer layer, improving electron transport to the cathode and cell efficiency.
3. Organic Field-Effect Transistors (OFET):
- Electron transport layer, improving performance and switching characteristics of the devices.
The use of NCEIL-4 as both an EIL and n-dopant for the electron transport layer offers several advantages. By replacing traditional materials like Liq, NCEIL-4 has been shown to extend device lifetimes by as much as 15 times, a significant leap in durability. Additionally, this compound lowers the charge injection barrier and lowers driving voltage, leading to energy consumption reduction in blue diodes. What makes this advancement particularly important is that, despite these improvements in efficiency and longevity, the spectral characteristics of the devices remain unaffected.
This is not all. Based on initial research on NCEIL-4 the market can expect an entire new family of products dedicated to enhancement of OLED stack and blue dopant coming live soon.